<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>query</title>
<link rel="stylesheet" href="../../boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
<link rel="home" href="../../index.html" title="Asio">
<link rel="up" href="../reference.html" title="Reference">
<link rel="prev" href="prefer_result/type.html" title="prefer_result::type">
<link rel="next" href="query_result.html" title="query_result">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../asio.png"></td></tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="prefer_result/type.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="query_result.html"><img src="../../next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="asio.reference.query"></a><a class="link" href="query.html" title="query">query</a>
</h3></div></div></div>
<p>
        <a class="indexterm" name="asio.indexterm.query"></a>
      </p>
<p>
        A customisation point that queries the value of a property.
      </p>
<pre class="programlisting"><span class="keyword">constexpr</span> <span class="identifier">unspecified</span> <span class="identifier">query</span> <span class="special">=</span> <span class="identifier">unspecified</span><span class="special">;</span>
</pre>
<p>
        The name <code class="computeroutput"><span class="identifier">query</span></code> denotes a
        customization point object. The expression <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span>
        <span class="identifier">P</span><span class="special">)</span></code>
        for some subexpressions <code class="computeroutput"><span class="identifier">E</span></code>
        and <code class="computeroutput"><span class="identifier">P</span></code> (with types <code class="computeroutput"><span class="identifier">T</span> <span class="special">=</span> <span class="identifier">decay_t</span><span class="special">&lt;</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">E</span><span class="special">)&gt;</span></code>
        and <code class="computeroutput"><span class="identifier">Prop</span> <span class="special">=</span>
        <span class="identifier">decay_t</span><span class="special">&lt;</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">P</span><span class="special">)&gt;</span></code>)
        is expression-equivalent to:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            If <code class="computeroutput"><span class="identifier">is_applicable_property_v</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Prop</span><span class="special">&gt;</span></code> is not a well-formed constant expression
            with value <code class="computeroutput"><span class="keyword">true</span></code>, <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code>
            is ill-formed.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput"><span class="identifier">Prop</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">static_query_v</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> if the expression <code class="computeroutput"><span class="identifier">Prop</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">static_query_v</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
            is a well-formed constant expression.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput"><span class="special">(</span><span class="identifier">E</span><span class="special">).</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">P</span><span class="special">)</span></code> if the expression <code class="computeroutput"><span class="special">(</span><span class="identifier">E</span><span class="special">).</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">P</span><span class="special">)</span></code>
            is well-formed.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput"><span class="identifier">query</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code>
            if the expression <code class="computeroutput"><span class="identifier">query</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code> is a valid expression with overload
            resolution performed in a context that does not include the declaration
            of the <code class="computeroutput"><span class="identifier">query</span></code> customization
            point object.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code>
            is ill-formed.
          </li>
</ul></div>
<h5>
<a name="asio.reference.query.h0"></a>
        <span><a name="asio.reference.query.requirements"></a></span><a class="link" href="query.html#asio.reference.query.requirements">Requirements</a>
      </h5>
<p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">asio/query.hpp</code>
      </p>
<p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="prefer_result/type.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="query_result.html"><img src="../../next.png" alt="Next"></a>
</div>
</body>
</html>
